For realizing an all-optical subscriber network, the PDS (Passive Double Star) technique has been considered to be useful and important, in which variety types of multiplexing schemes are used. In a PDS subscriber network system, it is required to use an optical receiver which performs AGC (Automatic Gain Control) or ATC (Automatic Threshold Control) to process burst mode optical signals. Such an optical receiver includes a level decision circuit, which decides the level of the input signal to generate a digital output signal of zero or one.
In a PON (Passive Optical Network) system, burst mode signals are transmitted from a transmitter to the optical receiver. For maintaining a high transmission rate in the PON system, it is required to start a level decision operation within the first several bits in the burst mode signal. According to a conventional level decision circuit, however, in order to maintain a high transmission rate, the structure of the circuitry becomes complicated. Further, the duty ratio becomes worse and an output error may occur frequently.